My biggest pet peeves are the spelling and grammar issues, and longwinded posts all in one paragraph. If it takes a lot of words to say what needs to be said, fine, but it's really irritating to have to wade through one ginormous paragraph.
I don't get too worked up about off-topic replies, as long as it's just a quick sidebar, and doesn't hijack the thread. I'm not that bothered by swearing, disguised or not, as long as it's not constant, and I'm not bothered by duplicate posts -- anyone can make a mistake.
Another one that I would have put on the list, though, is the exclusive use of large print and bright colour in a message: these should be used for emphasis, and it's possible to be really creative, but a whole message in large bright print is very irritating and I usually just don't bother reading it. Too bad -- the person might have had something very worthwhile to say, but I'll never know.
The biggie of them all, though, is the last one -- the poster who asks advice and then either ignores it or dismisses it argumentatively. As Susan said, if you didn't want to know, don't ask.